The present invention relates to an automobile-mount diesel engine and, more particularly to a diesel engine that is operated while switching between a premix combustion mode and a diffusion combustion mode.
Conventionally, diesel engines which carry out a switching control of combustion modes of fuel within an engine cylinder or cylinders according to an engine load are known. As the combustion modes of fuel, there are a diffusion combustion mode in which the fuel is combusted while injecting the fuel into a cylinder and a premix combustion mode in which the fuel is mixed within the cylinder before the fuel is lit. When the engine is under a high load, the diffusion combustion mode is used, and when the engine is under a low load, the premix combustion mode is used.
For example, JP2007-162544A discloses, in order to suppress generation of nitrogen oxide (NOx), soot, and noise (NVH: Noise Vibration Harshness) while smoothening the switching of the combustion modes, that a timing and amount of fuel being injected into a cylinder, and an oxygen concentration within the cylinder are continuously varied when shifting the combustion mode. Particularly, in the control disclosed in JP2007-162544A, during a transient state of the shifting of the combustion mode, a shift course from a fuel injection timing and an oxygen concentration before the shift to a fuel injection timing and an oxygen concentration after the shift is set so that generating amounts of NOx, NVH, and soot do not exceed tolerance limits, respectively. In addition, both the fuel injection timing and the oxygen concentration are changed gradually in accordance with the course.
However, depending on conditions upon the switching of the combustion mode, for example, if a difference of the oxygen concentrations within the cylinder before and after the switching is large, when both the fuel injection timing and the oxygen concentration are changed continuously, the oxygen concentration within a combustion chamber may become superfluous and the combustion noise (i.e., NVH) may exceed the tolerance limit, or the oxygen concentration within the combustion chamber may become thinner and the generation of soot may exceed the tolerance limit. That is, even if any shift course is set under the premise where both the fuel injection timing and the oxygen concentration are changed continuously, at least one of NVH and exhaust emission exceeds the tolerance limit and, thus, a shift course where both of them are avoided may be unable to be set. In this case, there is a problem in continuously changing both the fuel injection timing and the oxygen concentration as disclosed in JP2007-162544A.